DeLillo wrote a piece called "Notes Toward a Definitive Meditation (By Someone Else) on the Novel 'Americana'" for Epoch in 1972 (Vol 21, No. 3, pp. 327-329).
Here are a few excerpts:
Within the central narrative, the action moves forward while the protag's camera looks back, at least most of the time.
Eventually it becomes clear (or almost clear) that the book is being "written" in the year 1999. The narrator-protagonist is living on a small island where he contemplates his celluloid adventures as a young man. He uses as source material not only his mental recollections of that period but the movie he began making while traveling west and finished during a period of time merely alluded to in the book. The time frame narrows and widens from 1999 to the 1960's, from island to continent, from movie-being-made to the events which inspired the movie.
Many ideas, themes and characters were struck from the hulking manuscript (higher than a small radio) as the author blundered his way through the process of turning out what is called a publishable work.
Subtext 1: Patriotism as incest
Much of this survives in the final text. The author evidently constructed two planes of incest in 'Americana.' One is based on relations (or near-relations) between the protagonist and his mother. The second might be called political incest--the notion that baseless patriotism is an elaborately psychotic manifestation of love for mother country.
When protag goes to bed with a particular (older) woman, the two planes become one. ("...and entering her I was occupied by her, another turning on an axis, wrong way on the bed, the army occupied by the city...").